After five years of painful belt-tightening, the world's biggest gold miners are starting to cautiously loosen their purse strings and spend more money to find new deposits and build mines.

Several miners said this week that they have hiked budgets for exploration, construction and expansion projects. The plans, detailed in financial reports and conference calls, come as top producers face a "production cliff", with some analysts forecasting a sharp drop in their output in just four years due to under-investment.